Variables, Loops & Conditionals

#!/bin/bash
set -euo pipefail  # strict mode: exit on error, undefined vars, pipe fails

# Variables
NAME="World"
COUNT=42
GREETING="Hello $NAME, count is $COUNT"
echo "$GREETING"

# Command substitution
DATE=$(date +%Y-%m-%d)
FILES=$(ls *.txt 2>/dev/null | wc -l)

# String operations
STR="Hello World"
echo ${#STR}            # length: 11
echo ${STR:0:5}         # substring: Hello
echo ${STR/World/Bash}  # replace: Hello Bash
echo ${STR,,}           # lowercase: hello world
echo ${STR^^}           # uppercase: HELLO WORLD

# If/else
if [[ -f "$FILE" ]]; then
    echo "File exists"
elif [[ -d "$DIR" ]]; then
    echo "Directory exists"
else
    echo "Not found"
fi

# String comparison
[[ "$str" == "hello" ]]   # equal
[[ "$str" != "hello" ]]   # not equal
[[ -z "$str" ]]           # empty
[[ -n "$str" ]]           # not empty

# Number comparison
[[ $a -eq $b ]]   # equal
[[ $a -gt $b ]]   # greater than
[[ $a -lt $b ]]   # less than

# Loops
for file in *.txt; do
    echo "Processing: $file"
    wc -l "$file"
done

for i in {1..10}; do echo $i; done

while read -r line; do
    echo "Line: $line"
done < input.txt

# Array
FRUITS=("apple" "banana" "cherry")
echo ${FRUITS[0]}        # apple
echo ${FRUITS[@]}        # all elements
echo ${#FRUITS[@]}       # length: 3
FRUITS+=("date")         # append